The outstanding advantages of the known planetary assemblies having wet disk brakes is experienced in conditions where the braking force is applied--instead of braking the hub directly--to the sun wheel of the planetary assembly driving the hub or to the axle-shaft driving the sun wheel. In these cases the braking torque acting on the multiple disk brake mechanism is reduced in the ratio of the planetary mechanism. A further advantage of such mechanisms results from limiting the disturbing effects of the polluted environment usually associated with heavy duty vehicles, said mechanisms being immersed in the oil space of the axle. But also the utilization of oil filling of the axle for cooling the brake offers advantages.
It is expedient to mount the mechanism of such kind into the hub of the axle, and once the planetary drive is accommodated here, also the brake assembly is integrated with this mechanism. In exceptional cases, such axle arrangements--called sometimes adjustable bar type axles--are applied, where hubs, in a conventional sense of the word, are not provided. In such cases the planetary assembly is arranged in the middle portion of the axle, so that expediently also the brake assembly is integrated with the middle portion.
Examples of such known brake assemblies are described, among others in the U.S. Pat. Nos. 3,754,625, 4,037,694 and 4,646,880. The essential advantages mentioned above are characteristic of all such mechanism, but unfortunately they have also certain unfavourable properties that can be summarized as follows:
Because the hub mechanism is composed of two main assembly units (planetary mechanism and brake mechanism), the drawbacks of designs known from the art are concentrated in these two main fields or, in several cases, result from the unfavourable interaction of these two units.
1) As regards the useful life period, the load carrying capability and silent running of the planetary mechanism, it is considered to be favourable if the meshing of the planetary gears occurs in optimum position and--by assuming proper dimensions--the gears are capable of correcting inexactitudes due to manufacturing deficiencies and deviations caused by deformations arising under different load conditions of the planetary gear and running gear. Furthermore, from the point of view of the planetary gears, it is also essential to relieve the gears of loads resulting from the continuously changing reaction forces of brake disks and brake pistons because these effects too, tend to impede the gears in assuming their optimum meshing position. The middle one of the three gears (sun gear, planetary gear, ring gear), i.e. the planetary gear is in fixed position in all of the mentioned cases, the planetary carrier being firmly bolted to the hub. Hence, in optimum designs, both the sun gear and ring gear are capable of self-alignment to an extent overriding the sum of manufacturing errors and maximum deformation. So in that case both the sun gear and the ring gear can adapt themselves to the planetary gears.
2) For a steady operation silent running and long service life of the brake, it is considered favourable that the number of those elements in the planetary mechanism which exert a force on the components of the brake is as small as possible and the power-transmission chain in said brake mechanism is as short as possible. Moreover it is especially harmful if some loose linkage is present in the brake mechanism (e.g. a loose meshing between the ring gear support member and the ring gear), because this tends to cause unsteady operation, resonance and noisiness of the brake.
The main drawbacks of the solutions known from the art can be summarized as follows.
The solution shown in the U.S. Pat. No. 3,754,625 cannot be realized in a form which satisfies the condition described under item 1 outlined above, because the ring gear is not self-aligning. But there are further disadvantages of this arrangement: it is provided with only one internal splined brake disk, the disk wear cannot be measured and the ring gear cannot be replaced without disassembling the brake assembly. Neither the brake, nor any element of the planetary mechanism can be replaced without putting the vehicle on blocks or dismantling the wheel.
U.S. Pat. No. 4,037,694 is silent whether the ring gear is fitted to the ring gear support member with a clearance fit or a close fit.
But neither of these fits can be considered as advantageous because on the one hand if the splines fit close, the ring gear cannot adapt itself to the planetary gear, so the planetary mechanism will fail to function perfectly. On the other hand, with a loose fit both the external splined brake disks and the planetary gears tend to dislocate the ring gears.
So, in the mechanism the ring gear cannot adjust itself to the planetary gears and, due to the loose fit, the brake splines will chatter. Further drawbacks are that the spline wear cannot be measured and the ring gear cannot be replaced without unlocking the brake.
It is still another disadvantage of the above solution that for replacing the brake retainer spring or brake piston the spindle end nut has to be dismantled, and for doing so the axle must be put on blocks.
The main drawbacks of the arrangement described in the specification of the U.S. Pat. No. 4,646,880 are the following:
i) The ring gear being fixed, it cannot align itself with its gearing to the planetary gears.
ii) For replacing the ring gear the vehicle has to be placed on blocks and the entire wheel hub has to be disassembled together with the brake mechanism.
iii) Disk wear cannot be measured.
iv) The stationary disks bear against the rotary planetary carrier, so the entire brake mechanism performs a wobbling motion corresponding to the runout of the planetary carrier, moreover the brake piston imposes an axial load on the wheel bearings as well.